It has previously been proposed, as in U.S. Pat. No. 4,641,005, to increase the browning or crisping of the exterior of food as packaged in a disposable coated cardboard paper food container, by laminating to the surfaces of the container that abut the food packaged therein, a metallized electrically conductive layer, such as vapor-deposition metallized polyester or other thin plastic sheet and the like. The interaction of the microwave energy with the composite of the metallized plastic film or sheet causes additional heating adjacent the abutting exposed surface of the food which has been found to aid in browning and making the food more cosmetically compatible with other types of oven cooking, as well as better tasting.
Other techniques, including for localizing the supplemental heating are described, for example, in U.S. Pat. Nos. 4,735,513 and 4,878,765.
In some areas, the technique above described has attained the descriptor of a "susceptor film," usually constructed of polyester film, such as 46-gauge Dupont "Mylar" with a control density vapor-deposited aluminum coating measuring about 0.25 optical density or 65-100 ohms per square in electrical resistance. Such metallized films are then laminated to either paper or paperboard, as in the container or receptacle construction of disposable or foldable box containers. These have proved quite satisfactory for the microwave cooking of foods like popcorn, but have required techniques for overcoming seriously uneven and uncontrolled heating that have rendered such devices less than satisfactory for producing reliably reproducible results with other foods. Such susceptor films, as above stated, however, are now being used by the food industry for such purposes as browning and crisping foods, operating, however, as relatively uncontrolled resistive heaters because of their construction and inherent limitations of the conductive electrode and substrate structures.
To mitigate against the deleterious effects of localized hot zones and the like, techniques have been proposed for demetallizing, as by etching, different areas to reduce the generated heat and/or eliminate it in the specified zones, thereby to create more uniform browning or crisping by such patterned demetallization in the composite of the plastic film with its metallized layer. Such techniques are described, for example, in U.S. Pat. Nos. 4,865,921 and 4,959,120. The use of outer layers of lacquer to be printed in patterned heat zones has also been proposed, as in U.S. Pat. No. 4,963,424.
Another approach at patterned metalization for temperature control using electrodeposition techniques is described in U.S. Pat. No. 4,962,293. A foil grid used in association with a susceptor film has also been proposed in U.S. Pat. No. 4,927,991.
More recently, other types of membrane susceptor films have been proposed, embodying two metallized coatings in which at least one electrode has been selected for its microwave absorbing qualities and at least another metallized coating selected for its capability for controlling the level of transmitted energy--such being offered under the trademarks "Accucrisp," by A.D. Tech Advanced Dielectric Technologies, Inc., of Tauton, Mass.
Up until the present time, however, it has been considered essential to laminate and secure throughout its surface the metallized plastic film or other layered laminated composite of metallic layer or film with polyester or other suitable baking sheet and high-temperature thin plastic or paper, laminated as an integral unit throughout the paper or other wall of the container, carton or receptacle for the food; and laminating techniques have been developed for insuring such coextensive support of the metallized film by the carton or container wall and the lamination into such integral structure.
Underlying the present invention, however, is the discovery that the non-uniformity of microwave interaction with the metallized layer (and thus the incomplete browning or crisping heating over the layer) appears to result from this integration or total lamination of the film against the carton walls.
The present invention, to the contrary, steers away from this standard practice of lamination integration by bonding the film to the carton wall or other container at restricted and minimal regions only, such as the peripheral border of the metallized plastic film, thus securing the same to the container wall but in such a way as to incorporate an air pocket space between a substantial unlaminated major area of the container wall, over which the film is free to flex in response to hot air convection effects within the air pocket.